Stock Options and Other Equity-Based Incentive Instruments - Stock Option Activity (Detail)	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding, Number of Shares, beginning balance | shares	592,250
Options granted, Number of Shares | shares	0
Options exercised, Number of Shares | shares	(145,850)
Options forfeited, Number of Shares | shares	0
Options outstanding, Number of Shares, ending balance | shares	446,400
Options outstanding, Weighted Average Exercise Price, beginning balance	$ 17.12
Options granted, Weighted Average Exercise Price	0
Options exercised, Weighted Average Exercise Price	17.97
Options forfeited, Weighted Average Exercise Price	0
Options outstanding, Weighted Average Exercise Price, ending balance	16.84
Minimum [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding, Option Price per Share, beginning balance	12.63
Options granted, Option Price per Share	0
Options exercised, Option Price per Share	14.54
Options forfeited, Option Price per Share	0
Options outstanding, Option Price per Share, ending balance	12.63
Maximum [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding, Option Price per Share, beginning balance	23.21
Options granted, Option Price per Share	0
Options exercised, Option Price per Share	22.54
Options forfeited, Option Price per Share	0
Options outstanding, Option Price per Share, ending balance	$ 23.21